FIG. 1 shows an illustration of an exemplary wideband code division multiple access (WCDMA) architecture 10 including a core network 12, a radio access network (RAN) 14 and a plurality of user terminals 16, also referred to as user equipment (UE). The RAN includes one or more components 18 responsible for radio network control (RNC) and one or more base station components 20, also referred to as “Nodes B”, that mainly perform air interface processing. Each base station component 20 serves one or more network cells. One RNC component 18 and one or more associated base station components 20 constitute a radio network subsystem (RNS). A RAN typically comprises a plurality of such RNSs.
Enhancements in the uplink direction of WCDMA are currently being standardised within the 3rd generation partnership project (3GGP). Among the various standardized features are fast scheduling and fast hybrid automatic repeat request (HARQ) as described in the technical specification TS 25.309 “FDD Enhanced Uplink”.
Conventional radio resource management techniques include features such as admission and congestion control (ACC), radio link control (RLC), and outer loop power control (OLPC). As shown in FIG. 1, these features are conventionally located in the RNC component 18. On the other hand, the new features introduced for enhancing the uplink direction, such as fast scheduling and fast HARQ, are located in the base station components 20.
The technical specification TS 25.309 not only introduces new control features but also new uplink channels. In addition to conventional uplink channels such as the dedicated physical data channel (DPDCH) and the (high speed) dedicated physical control channel ((HS-) DPCCH), an enhanced DPDCH (E-DPDCH) and an enhanced DPCCH (E-DPCCH) are introduced as shown in FIG. 2. The DPCCH carries pilot symbols and portions of the outband control signalling. Remaining outband control signalling for implementing the enhancements in the uplink direction is carried on the E-DPCCH, while the E-DPDCH carries the data transmitted using the enhanced uplink features. According to the technical specification TS 25.309, the term E-DCH generally denotes a new dedicated transport channel type or enhancements to an existing dedicated transport channel type. In this connection, an E-DCH active set, or simply active set, designates the set of cells which carry the E-DCH for one particular user terminal.
In the following, the feature of fast uplink scheduling will be discussed in more detail. Generally, fast scheduling as used in the uplink context here denotes the possibility for a base station component 20 to control when a user terminal 16 is transmitting and, in combination with adaptive modulation and coding (AMC), at which data rate.
Using the fast scheduling feature, the base station component 20 sends a resource indication (“scheduling grant”) in the downlink to the user terminal 16. The scheduling grant indicates to the user terminal the maximum amount of uplink resources the user terminal is allowed to use. The scheduling grants are used in connection with the E-DCH transport format combination (TFC) selection and control the maximum allowed E-DPDCH/DPCCH power ratio. In general, the scheduling grants set an upper limit on the data rate a particular user terminal may use. However, the power situation in a particular user terminal, as well as activity on other, non-scheduled channels, may lead to the situation that the user terminal transmits with a lower data rate on the E-DCH than that indicated by means of the scheduling grants.
The scheduling grants can be divided into absolute grants on the one hand and into relative grants on the other. By using these two types of grants, the scheduling base station component can control the transmission behaviour of each individual user terminal.
Absolute grants are used to set an absolute limitation (in terms of power ratio relative DPCCH) for the maximum amount of uplink resources the E-DCH may use for data transmission. The maximum amount of uplink resources allowed for E-DCH data transmission determines the maximum data rate on E-DCH. Typically, absolute grants are used for significant but infrequent changes of the resource allocation for a particular user terminal (e.g. at times of bearer setup or when granting resources in response to a scheduling request received from a user terminal).
Generally, there is only a single E-AGCH for all user terminals that are served by a particular cell. Absolute grants are sent by the E-DCH cell serving a particular user terminal and transmitted on a control channel called E-AGCH (E-DCH absolute grant channel) that can be shared by multiple user terminals.
Relative grants on the other hand are used to update the resource allocation for a particular terminal. Relative grants can be sent by serving as well as non-serving base station components and typically as a complement to absolute grants. A relative grant from a serving cell can take one of three different signaling contents, namely either “up”, “down” or “hold”. A relative grant from a non-serving cell can take one of two different values, “down” or “hold”. These signaling contents refer to uplink resource limitations associated with a user terminal relative to the amount of resource the user terminal is currently using. Relative grants are transmitted on individual control channels, namely on E-DCH relative grant channels (E-RGCHs). FIG. 3 shows a schematic illustration of E-RGCH und E-AGCH signaling.
There is one E-RGCH per user terminal from the serving cell, and each user terminal may receive one relative grant per transmission time interval (TTI). Thus, the relative grants have some similarities with power control instructions.
In a soft handover scenario, in which a user terminal is communicating with a plurality of cells, the user terminal receives absolute grants only from a single one of these cells, namely from the serving E-DCH cell (or simply serving cell). The serving cell has therefore the main responsibility for the scheduling operation. However, also non-serving cells involved in a soft handover with a particular user terminal are able to influence the resource consumption of this user terminal in order to control the overall interference level within their own cell coverage. In this context, a particular user terminal may receive relative grants from both the serving cells and all non-serving cells involved in a soft handover with the particular user terminal.
A serving E-DCH radio link set (or simply serving RLS) denotes the set of cells which contains at least the serving cell and from which the user terminal can receive relative grants and absolute grants. Each user terminal has only one serving RLS. A non-serving E-DCH RLS (or simply non-serving RLS) denotes the set of cells which does not contain the serving cell and from which the user terminal can receive absolute grants. A user terminal may have zero, one or several non-serving RLSs.
Base station components of the non-serving RLS will only send relative grants to the user terminal. The relative grants from such base station components are restricted to the value “down” and “hold”. In the absence of a “down” from any non-serving RLS, the user terminal simply follows the serving RLS's scheduling grants.
If a user terminal is receiving a “down” from any non-serving cell, this is an indication that the cell in question is overloaded and the user terminal shall therefore reduce its data rate compared to the data rate it is currently using (even if one or more grants from the serving cell suggest an increase). Thus, the relative grant from a non-serving cell serves as an overload indicator. The overload indicator is sent to all user terminals for which the overloaded cell is a non-serving cell as shown FIG. 3.
In addition to scheduling grants, the fast scheduling scheme further includes resource demands (“scheduling information”) that can be issued by user terminals to request radio resources. By means of such scheduling information, a user terminal may indicate its current status (e.g. to provide an indication of its buffer status, traffic priority and power availability) as shown in FIG. 3. The scheduling information can be exploited by the scheduling base station component in its scheduling decision.
The scheduling information is sent in the same way as data transmissions (i.e. on the E-DCH) and thus benefit from the gains of HARQ with soft combining. Even if the user terminal has no scheduling grant and is therefore not allowed to transmit any user data on the E-DCH, the user terminal is still allowed to transmit scheduling information inband. In addition to the inband scheduling information, there is a single “happy” bit included in the uplink outband control signalling sent on the E-DPCCH. This happy bit is used to indicate that the terminal supports and would benefit from a higher data rate.
It has been found that in conventional cellular communication systems including uplink scheduling, the radio resource management could be improved due to an insufficient overall coordination of the radio resource management functions for the uplink direction. Accordingly, there is a need for an improved radio resource management technique.